Some components must be connected in series. For example, the sender in the fuel tank varies its resistance according to the amount of fuel in the tank, and sends a small electrical current to the fuel gauge.
There are many fuses, each protecting a small group of components, so that one blown fuse does not shut down the whole system. Many of the fuses are grouped together in a fuse box, but there may also be line fuses in the wiring.
The two components are connected in series so that the varying resistance in the sender will affect the position of the needle on the gauge.
Most car handbooks and service manuals include a wiring diagram which can be difficult to follow. The colour-coding, however, is a useful guide to tracing wiring.